Expanding the expression we get
(2)(4)+(2)(-j5)+(j3)(4)+(j3)(-j5)
Next we multiply the 4 products, pretending that j is an actual number.
8-j10+j12-j215
We replace j2 by -1.
8-j10+j12-(-1)15
Then we gather the real and imaginary terms.
(8+15)+j(12-10) = 23+j2
The result is the sum of the real number 23 and the imaginary number j2.